Inductive tuner



June 27, 1939. WARE 2,163,645

4 INDUCTIVE TUNER Filed Augf8, 1936 3 Sheets-Sheet l INVENTOR Hm/War ATTORNEY June 27, 1939. P. WARE nmucuva TUNER Filed Aug. 8, 1936 3 Sheets-Sheet 2 INVENTOR )Zw/ fifire BY ATTORNEY June 27, 1939. P. WARE ,153,645

INDUCTIVE TUNER Filed Aug. 8, 1936 3 Sheets-Sheet 3 mHIllW lNVENTO R far/l 2 am BY m ATTORN EY Patented June 27, 19,39

PATENT OFFICE INDUCTIVE TUNER Paul Ware, Indianapolis, Ind., assignor to P. R.

Mallory & 00., Inc., Indianapolis, Ind., a corporation of Delaware Application August 8,

17 Claims.

This invention relates to tuning devices and more particularly to variable inductances for tuning radio receivers and the like.

This application comprises a continuation in 5 part of my application for Radio apparatus Serial No. 31,823, filed July 17, 1935.

An object of the invention is to provide an improved tuning device.

Another object is to improve the structure of a variable inductance device.

A further object is the provision of a tuning arrangement and apparatus for radio receivers which Will allow stepless tuning over an extensive frequency range.

Other objects of the invention will be apparent from the following description and accompanying drawings taken in connection with the appended claims.

The invention comprises the features of construction, combination of elements, arrangement of parts, and methods of manufacture and operation referred to above or which will be brought out and exemplified in the disclosure hereinafter set forth, including the illustrations in the drawings, the scope of the invention being indicated in the appended claims.

For a fuller understanding of the nature and objects of the invention as well as for specific fulfillment thereof, reference should be had to the following detailed description taken in connection with the accompanying drawings, in which:

Figure l is a top view, partly in section, of a tuning device arranged according to one form of the present invention;

35 Figure 2 is a section on the line 2-2 of Figure 1;

Figure 3 is a section on the line 3-3 of Figure 2;

Figure 4 is a section on the line 44 of Fig- 40 ure.

Figure 5 is a detailed view illustrating the guide trolley for the slide contact and also illustrating an improved means of making the coil form;

Figure 6 is a detail view illustrating the contact between the slide and the wire coil;

Figure '7 is a detailed exploded view of the contact carriage with parts broken away;

Figure 8 shows a modification of the tuning device comprising two ganged variable inductances;

Figure 9 is a face view of a dial arrangement adapted for use with the tuning device;

Figure 10 is a section on the line llll of Figure 9;

1936, Serial No. 94,927

Figure 11 is a section on the line IIH of Figure 10;

, Figure 12 is a detail view of certain parts of a modified contactor structure;

Figure 13 is another detail view of the modified structure; and

Figure 14 is a section through a modified trolley wheel.

While a preferred embodiment of the invention is described herein, it is contemplated that considerable variation may be made in the method of procedure and the construction of parts without departing from the spirit of the invention. In the following description and in the claims, parts will be identified by specific names for convenience, but they are intended to be as generic in their application to similar parts as theart will permit. I

In providing a variable inductance device which will operate satisfactorily for low and high fre- I quencies, a number of problems are encountered, some of a mechanical nature and others principally electrical in nature. For instancejshielding of, parts is important, both from an electrical standpoint and as structural protection for the unit against dirt, dust and moisture. The apparatus must be so constructed as to insure satisfactorily long life. Also of importance is the requirement that the device be easy of manipulation. constant in response and capable of satisfactory operation even when used by a person entirely unfamiliar with the apparatus.

The present invention not only meets the above requirements but provides a greater frequency tuning range than that obtainable by prior meth- 36 ods such as by the use of variable condensers, without extensive switching from one frequency band to another. The structure employed makes possible the tuning of a radio receiving set to any frequency' in present day broadcast and 40 short wave bands without the requirement of more than two switching positions. It is possible, in fact, to obtain a frequency tuning ratio of 11 to 1 without switching. The structures herein described are suitable, for example, for use in cir- I cults such as those disclosed in my application Serial No. 94,928 for Tuning circuit filed concurrently herewith.

Referring to the drawings, the variable inductanee device of the present invention may comprise an inductance coil l0 mounted on a rotatable form ll of insulating material. The coil I0 and the form' H upon which it is mounted are supported for rotation by a rotatable shaft I2 5 material, such as Bakelite or ceramic.

. and a metal ring 35 by nut 35.

which is rotated in bearings it and it mounted in end plates 55 and iii of the unit.

The case of the unit, which also serves as an electrical shield, comprises a bottom plate I! having turned down edges and parallel upstanding end plates 55 and it which-are secured to the end flanges iii of the bottom plate 5? by machine screws it. The upper corners of end plates and it are rounded oh and a correspondingly rounded cover 25 fits over the edges of end plates i5 and it to provide a top and side for the unit. Cover 2i] is provided with flanges 25 which press against the outer faces of end plates i5 and it and is also provided with a pair of hoot; flanges 22 and 23 adapted to hook under the side flanges on bottom plate if.

The cover 2% is secured in position in the foliowing manner: Hook flange 22 on cover 20 is first'hoolred under flange 2 3 on bottom plate I l and the rest of cover 28 is fitted over the edges of end walls i5 and ft. Bottom plate l! is prowith an anchoring plate 25 secured along one edge of its lower surface by screwsif25. Rivets can, or" course, be used in place of the screws. Plate 25 is provided with a down turned flange 27 along its outer edge and flush with the edge of plate Bl. Plate 25 is provided, near its outer edge, with a pair of tightening screws 28, the ends of which rest in recesses 29 in bottom plate It.

After cover 28 has been fitted over the unit,

flange 23 is hooked under the edge of flange 27 on anchoring plate 25 as indicated in Figure 3. Screws 28 may then be tightened so as to spread plates it and 25 apart as indicated in Figure t. This results in a tightening of cover 20 over the unit and it will thereby be held securely in placeand can only be removed by loosening screws 28. The tightening insures a good, permanent electrical contact between the shield parts IT and 20 and also renders the mechanism dust proof.

The shaft" 32, upon which coil form H is supported for-rotation, passes through end plates i5 and it anolmay extend beyond them at each end. A knob 3i? may be secured to one end of shaft i2 so that it may be rotated manually or other rotating means may be provided. In a radio set knob 35 may be the control knob provided on the panel of the radio set for tuning to the various frequencies at which signals are received. The bearings i3 and i for shaft i2, secured to plate i5 and it, respectively, may comprise ball bearing assemblies having ball bearings 35. The shaft is of hard copper. An insulating sleeve MS may be provided at one end of the shaft, asshownvto insulate it from the bearing and thus prevent any electrical noise which might otherwise be caused due to slight potential differences induced by the radio frequencies.

Coil form ii is a hollow cylinder of insulating It is rigidly secured for rotation with shaft l2 by end disc 32 of insulating material. A threaded sleeve is secured to shaft i2 by set screws 35. End disc 32 is clamped between a shoulder on sleeve Form Ii is tightly secured against the face of disc 32 by a plurality of eyelets 37 and spade bolts 38 held by nuts fill as indicated in Figure 2.

A slip ring 39 is mounted on the outer face of disc 32 and supported in spaced relation thereto by a plurality of legs lEi on ring 39. Legs 40 are held on spade bolts 38 by nuts l.

A V-shaped groove is cut in form H in which the wire is tightly wound. The shape of the lgroove is shown in Figure 6. It will be noted 5:1 greases that the angle between the two sides of the groove is 90. The groove has been cut to have a constant distance from the axis of the form H. The wire rests on the sides of the groove. By this arrangement any eccentricity in the form will not result in eccentricity in the coil and hence more uniform coils may be constructed.

In the preferred method of applying the wire to the form it is preferably first attached at one end to lug 33 on ring 35. The wire is carried through the inside of cylindrical form I! to its open end 44 where it is passed through nick A5 to the outside of the form and is there wound in groove 42 machined in the outside surface of form l i. Care is exercised to wind coil tightly in place and when the coil has been completely wound it is looped around eyelet (or rivet) 46 located near the end of form II and the free end of the wire is then soldered to one of the legs 40 of slip ring 39 as indicated at 41.

It will be noted that the pitch of coil winding I is not constant but is greater at the right- .hand end of the coil as shown in Figure I than at the left-hand end. With a coil 3 inches long and 1% inches in diameter approximately 36 turns may be provided, for example, of which 7 or 8 turns may have a pitch of /8 inch and the remainder of the turns a pitch of inch. Thus, slightly more than one-third of the coil may have a wide pitch and the remainder a narrow pitch The variation in pitch of the coil enables easier tuning of the inductance when only a very few turns at the right-hand or high frequency end are in the circuit and tends to equalize the percentage change in inductance for a given rotation of the coil over its length.

The coil in is preferably formed of hard drawn copper wire of circular cross section plated with a smooth uniform closely adherent layer of silver. It is possible, however, to use other base metals such as hardened copper alloys or other highly conductive metals and other platings may be substituted for the silver such as for example rhodium plating. When silver plating is used, it is processed with a thin lanum oil solution to retard sulfiding and oxidation. This insures a permanent bright and untarnishable conducting surface on the wire over which the contactor may slide. By applying the silver plating to a hard wire base, such as hard drawn copper wire the tendency toward working and cracking of the silver is reduced to a material extent so that a considerably longer life is obtained.

The number of turns of coil in which are conneoted in the tuning circuit is controlled by contactor 58 which makes sliding contact with coil l0 and contact bar 89 parallel to a side of coil Contactor 38 comprises metal frame spring contactor 5| for making contact with contact bar it, spring contactor 52 for making contact with coil to and trolley wheels 53 and 54 of insulating material, for guiding contactor 52 along wire of coil ill. Metal frame 50 comprises a central table portion 6|] extending from which are a pair of angularly disposed portions integral their center as indicatedat 56 to provide bifurcated contact tips. This insures uniform contact at all times. However, the contact tips of spring 51 are sufilciently yieldable to allow the contactor 58 to adjust itself to the variations in pitch of the coil HI. Lock washer 51 is interposed between spring contact member 5! and table 60. It will be noted that there are, in effect, four individual contact surfaces between spring 5| and contact bar 45.

Trolley wheels 53 and 54 may be of molded Bakelite. Each wheel has a pair of spaced flanges 58 as seen most clearly in Figure 5. Flanges 58 are spaced apart by a distance slightly greater than the diameter of the wire of coil 10. The depth of the grooves between flanges 55 is greater than the height of the wire of coil l0 above the surface of coil form II. By this arrangement the trolley wheels will clear the top surface of the wire and they are are thus prevented from disturbing the wire surface and its coating along the path travelled by sliding contact 52. Thus wheels 53 and 55may rest on the coil form with the edges of flanges 58 against the surface of the form as indicated in Figure 5 and the trolley wheels 53 and 54 will be guided by the wire.

The trolley wheels are; each provided with a metal spindle 59 which may be molded into the insulating material of the wheel; .the wheels are each mounted in the aligned grooves formed by U-shaped portions 63 in side arms SI of frame 50. The ends of spindles 59 are inserted in pairs of bushings 64 which are held in the U-shaped portions 63 of the frame by spring clips55.

Spring clips 65 are each provided with flanges 56 and 61 bent into a box-like form so as to be adaptedto receive an end of frame 55. The spring clips are also each provided with a pair of integral leaf spring arms 58 which extend along the lower faces of arms 5| of the frame and retain bushings 64 in U-shaped portion 53 by spring action. Trolley wheels 55 and 55 are thus mounted for rotation in frame 50 and held in position by spring clips 55. Bushings 64 are adjusted to prevent excessive and play in the trolley wheels but to allow them to turn freely. In some instances spring clips 65 and bushings 64 may be dispensed with so that the spindles of the trolley wheels rest directly in grooves 53, held in position due to the relationship between the frame 50 and coil form ll.

Contactor spring 52 is riveted to the underside of table portion 50 by rivets 65 which pass through the spring, the table portion and an interposed spacer plate lll. Spring 52 maybe of phosphor bronze or other metallic spring material and is tapered to a bifurcated point 'H, slightly wider than the diameterof the wire of coil l0. Point H is bent into a U-shaped form so as to provide a convex surface in contact with the wire. The main body of contactor spring 52 is preferably tangent to the coil ID at the point of contact. The end H of contactor 52 is split along its center line at H2 for a short distance back from the point of contact to provide parallel, semi-independent contacting portions pressing against the surface of the wire of coil l0 (see Figure 6). The trolley wheel groove width and bearings are held to such maximum additive tolerances that contactor 52 is held in contact with an individual turn of the coil with both nibs riding thereon and in no instance is there contact to two succeeding turns at one time.

The double nibbed design of spring contactor 52 greatly minimizes the possibilityof noise introduced into the circuit by irregularities on the surface of the coil Hl. With a single contacting surface a slight hump or depression in the surface of the wire may frequently result in momentarily breaking the contact between the end of the spring and the wire at the moment the end jumps over the irregularity in the wire during tuning. With two parallel contact surfaces resting side by side on the wire as provided by the arrangement described the probability of a momentary break in the contact is very. greatly reduced since two parallel irregularities in the wire surface are not encountered in first-class plated and processed wire. If a minute surface irregularity causes a break in the contact between one of the nibs and the wire the other nib will serve to maintain the circuit closed. The two contacting nibs of spring 52 must slide in parallel along the length of the wire since they determine the amount of wire of coil III that is in the tuning circuit and hence the frequency of the circuit. With this arrangement therefore one nib may be raised without altering the tuning, whereas if the points of contact of the nibs were arranged sequentially along the wire only the one nearer the right end of coil I0 would determine the tuning frequency and the other nib would be inoperative.

The multiple contact advantage is also present in the contacts between spring 5| and track 49 as previously mentioned.

The stiffness and biasing of spring 52 are preferably so adjusted as to give a total contact pressure between the spring 52 and the wire of coil H) of approximately 50 grams, or '25 grams for each contact tip. This low pressure is made feasible by the uniform position in which the contactor spring is held relative to the wire. This pressure is found to be sufficient to insure a reliable contact with two sliding nibs and yet be well under a value that will cause excessive wear or siezing. As compared with the roller contact described in my application Serialv No. 31,823, the sliding contact has the advantage of tending to clean itself and the wire surface of minute irregularities as dust or chemical formations, whereas, the roller contact may often ride over them and cause noise.

In tests made with a coil of' hard drawn copper wire plated with silver and a phosphor bronze double-nibbed contact spring I have discovered that a pressure of 50 grams per nib causes excessive wear of the surfaces of the'conductors resulting in a tearing loose of particles of metal and rapid destruction of the unit. On the other hand a pressure of 10 grams per nib was insufficient to maintain reliable contact and tended to cause excessive noise. With pressures in the order of 25 grams ptr nib reliable contact was obtained and after 25,000 round trip operations of the device wherein the contactor travelled from end to end the device still functioned as intended and did not show excessive wear.

The thickness of spacer I0 is sochosen as to make spring 52 substantially tangent with coil II] in the vicinity of the contact, in order that the contact pressure shall be independent of the direction of rotation of the coil and that no danger of -mechanical oscillation of the contact spring during rotation of the coil shall be possible. By the arrangement shown the distance "I2 between the bifurcated tip Ilof spring 52 and table 80 will remain substantially constant during tuning. Otherwise there would exist the possibility of undue mechanical working of the spring metal resulting eventually in failure and breakage of the spring and excessive wear in the wire plating. Any eccentricity in the running of coil form II is compensated for by the flexing of spring 5| and not by flexing of spring 52. Only slight flexing of spring 52 is needed due to the small eccenthe same reason.

tricity existing between form II and coil Ill. Whereas considerable variations in pressure between spring 5I and the contact bar 49 are not detrimental because spring 5I slides a comparatively short distance along bar 49, it would result' in rapid failure of the contact and the wire of coil l0 if the pressure of spring 52 were to vary to any great extent. The pressure of spring 52 against the coil I0 must be relatively light in order to prevent excessive wear and disruptive action between the contact spring and the wire during use which may involve a total contact travel of several hundred miles. In order to keep down wear the spring pressure must be lighter than would normally produce reliability if the sliding contact were notaccurately positioned by the trolley wheels with respect to the wire at all limes and were not bifurcated so that one contact nib will remain in contact even if the other is momentarily lifted off.

Any slight tendencies of motion of the con-' tactor assembly 48, other than that required for operation, are largely confined to slight rotation of the contactor assembly about an axis located approximately at the center of the wire over which the trolleys and the tips of spring 52 are pressing. Such motion as may take place in this manner, however, is insufficient to permit either of the contact nibs to leave the wire or to cause grounded through bifurcated contact spring 13 which is secured to end plate I5 by screws Id. The split end of spring I3 makes contact with sleeve 33 which is secured to shaft I2. The other end of coil I0 is soldered to one of the lugs 48 of slip ring 39. Bifurcated spring I5 makes sliding contact with the circular face of slip ring 39. Spring "I5 is supported by strip I6 of sheet insulating material supported on plate I5 in spaced relatiouthereto by screws ll and sleeves 18. Spring "I5 is positioned between insulating strip 16' and end wall I5. Capacity losses between spring '15 and plate I5 are reduced by thus providing an air dielectric between the two parts. Slip ring 39 is spaced away from end disc 32 for Spring 15 is provided with a pair of angular flanges "I9 for stiffening purposes.

Adjustable end inductance coil is connected to the end of coil I0 through spring I5 to which one of the ends of coil 80 is soldered at I8I. Coil 80 is preferably formed of hard drawn copper wire of sufficient stiffness and resiliencyto return to its original shape if compressed or extended. Coil 80 is held between two buttons 8i and 82 of insulating material, each button being provided with a shoulder, as indicated, for receiving an end of the coil. The coil assembly is supported by a bracket 83 attached to the bottom ll of the metal case and a long brass screw 84 extends through the entire assembly and bottom I! of the case. Bracket 83 is internally threaded so that screw 84 may be tightened against the spring pressure of coil 80 by turning it with a screw driver. Both ends of screw 84 may preferably be provided with screw driver slots so that adjustment may be made from either end. In this way it is possible to delicately adjust the inductance of coil 80 by merelyturning screw 84 to bring the adjacent turns of the coil into closer or more widely spaced relationship. The flexibility of the end of coil 80 which connects to the spring at I8I is suflir-ient to accommodate adjustment of the coil as described. End inductance coil 80 is preferably made to have a higher Q than an equivalent inductive part of coil Ill and it is thereby possible to improve the tuning characteristics of the unit at the high frequency end.

Figure 8 shows a modified tuning unit in which two coils l0 and IIJa are mounted for rotation on a single long shaft I2a. The arrangement of parts is generally similar for each section of the unit shown in Figure 8 asfor the single section unit shown in the preceding figures. With this unit, however, the base plate Ila and cover 20a of the case are extended over the length of the combined unit and a partition 85 is interposed between the units. Partition 85 is provided with flanges 86 which rest against the sides of cover 20a, and is secured to base plate I'Ia, by flange 8'! and screws 88. The bearings I3 and I4 for shaft l2a are located at the two ends of the unit, the shaft passing through a clearance hole inpartition 85 without support at this central point. Figure 8 also shows a stationary coil 89 supported on coil form 98 within and co-axial with coil lfla. Coil form 90 may be supported from partition 85 by screws 9i passing through flange .92 on the end of coil form 90. One end of coil 89 is shown grounded to the partition 85 and the other end free to be connected as desired into the circuit. However, other types of coil connections may be provided, depending upon the circuit requirements. Coil 89, for example, may be the mutual coil referred to in my copending application 94,928, filed August 8, 1936.

Coils I9 and Illa in Figure 8 are each provided with an individual contactor 48 similar to that illustrated in the preceding figures. In this case both contactors slide along contact bar 49a which extends the entire length of the case and is soldered or welded in position to the two end plates and the partition.

A three or'four section unit may be made by extending the construction just described.

In the tuning of short-wave circuits it is desirable to employ a dial scale as long as possible for the reason'that in the shorter wave length regions the stations are easily passed and, hence, a lengthening in the dial scale to facilitate observation of the wave lengths being tuned and a pansion mechanism would be required to achieve accurate tuning. Compared with this, the inductive tuning system described herein, affords a primary tuning mechanism passing through several complete revolutions so that where a direct drive is employed between the tuner and the dial a longer scale can be obtained than could under any reasonable circumstances be devised for a condenser tuning arrangement. In fact, it becomes necessary with such an inductive tuning system to employ a step-up mechanism to reduce the dial scale length to a reasonable dimension.

Figures 9, 10 and 11 show details of a dial assembly suitable for use in an inductive tuning system of the type just described. Referring to the figures tuning knob 851s attached directly to the shaft 94 which may be the operating shaft of the device, such as shaft I2 in Figure 1. Pinion gear I22 is keyed to shaft 9| and drives gear pair I25, I26 which, with pinion I21, are attached together to rotate on shaft I23. In turn pinion I21 drives gear pair II8, I20 which are attached to sleeve 96 which has a running fit on drive shaft 94. Dial 81 is attached to the sleeve 86 by set screw 88 through the hub. On the back of dial 91 is attached a spiral track I00 of approximately three full turns as shown in Figure 11. The vertical frame I02 shown in Figures 10 and 11 is provided with slot I03 so that frame I02 may slide vertically over sleeve 86, and is additionally provided with a central slot at the bottom where it may likewise readily slide over guide I06. At the top of member I02 there is provided a shoe I04 and a leaf spring I05 which co-operatively engage and slide along the spiral track I00. It will be seen from Fig. 10 that the bottom of member I02 is bent forward and then upward to provide an attachment at I09 for a front vertical member I08 in whose center is a slot IIO which may slide over sleeve 98 guided by collar Ill. The top of vertical member I08 is disposed close to facing 88 of the dial and is provided with a window III and opposed indexing points II2 and H3. Facing 88 is of paper or other suitable material on which are scales I. I1 and I I8 calibrated to indicate the tuning frequency or wave length. The collar Ill is attached to sleeve 95 by means of set screw H5. The spacing between the front end of sleeve and the rear of knob 35 is such as to allow rea-- sonable play between the parts but may constrain the moving parts of the assembly from being thrust too far forward. It will be seen that the two connected vertical members I02 and I08 provide a yoke which moves up and down actuated by spiral guide I 00 which is rotated to the right or left whenever the knob is turned for tuning. The combined ratio of the two gear pairs I25, I26 and H8, I20 is such that for full motion of the dial spiral the contactor co-operating with the inductive tuner coil attached directly to shaft 94 will move from one end of the coil to the other. Curved portions IOI, IOI at the extremities of spiral guide I00 serve as mechanical stops to the entire mechanism. The gear pairs II3, I20 and I25. I26 are of such construction as to minimize backlash in operation. In this construction the front gear of each pair is designed to have a floating fit on the respective sleeves to 'which their companion gears are fixed and is spring coupled to its respective fixed gear by coil springs I2I held under tension therebetween in a well-known manner. It will be seen that with such a mechanism the scale may be viewed through window III and that the mechanism, in cooperation with the inductive tuner previously described enable accurate calibration and easy tuning to the exact frequency desired. In fact the greater the number of spiral turns on the dial the less can be the gear ratio between the dial and primary actuating shaft 84 and, consequently, the longer the scale and the greater its accuracy of calibration.

In some instances the number of turns on the dial may be sufllcient so that the gear ratio may become 1:1 and hence dispensed with, and the number of dial spirals equal the number of turns on the inductive tuner.

Figures 12 and 13 show an alternative method of axially guiding the contacting member along the coil I0 on'form II. Instead of the single contact bar 49 shown in Fig. 2, there are provided two contact bars.I38, I35 positioned with their contact edges on the straight line (marked "1! in Fig. 13) which runs through the points of mechanical contact of the insulating trolley wheels I3I, I3I with the coil form II. By this arrangement it is apparent that the axial thrust on the contact assembly'fi. e., parallel to the axis of form II) caused by the screw action of coil I0 when the coil form II is rotated is in the same plane as the frictional resistance components resulting from the light operating friction of springs I28, I28 sliding on contact bars I30, I 36. Thus this double contact bar arrangement makes it possible to eliminate any turning moment tending to rotate the contactor around line X. In other words the arm of moment of the frictional component about the line joiningthe points of contact of the flanges of wheels I3I with the coil form is reduced to zero. This advantage becomes of greater importance when larger structural units are made or when wheels I3I are of increased size. Since only limited end clearance is available the length of springs I28 must be kept at a minimum. In larger units, therefore, springs I28 cannot be correspondingly increased andthe resistance offered by springs I28 to rotation about the line X is less. The use of double contact bars also further insures good contact and increases the mechanical strength.

The upper part of Fig. 12 shows the details of the contactor bar I36. The end flanges I31, one of which is not shown, may be pressed in one operation from sheet material. Bar I36 is silver plated and processed with lanum oil solution to retard sulfiding and oxidizing.

The contactor assembly shown in part in the lower part of Fig. 12 comprises frame I21 having grooved openings for mounting wheels I3I and springs I28 mounted on its ends. Ears I28 of spring I28, determine the proper position of spring I28 with respect to member I21 in assembly and additionally tend to prevent any rotation of the spring member. The channelled extremes of spring I28 are beaded as at I30, I30 to provide short, low friction contact portions with the smooth bottom of bar I36.

It will be noted that in the arrangement of Figure 13 the separate bushings as well as the contact of spring member I35 is provided with an ofiset portion at its upper extremity which obviates the necessity of aspacer such as shown in Fig. i.

Fig. 14 shows a detail of one of the insulated wheel assemblies having a metal pinion I32 with large cylindrical central portion l34, around which the grooved wheel l3l of insulating material is pressed or moulded and turned-down pinion ends I33. The insulating material may either be pressed or moulded onto the central portion B36 and a preferred mode of construction is .to knurl the cylindrical part I34 and mould Bakelite or other insulating material onto it. The desired groove is preferably cut in the periphery of wheel |3i after molding to insure a smooth groove although in some instances the groove may be moulded. Contact spring I35 has its main body portion substantially in the line tangent of coil ill at the point of sliding contact.

The device described above is suitable for use as a variable inductance tuner in a radio set and in such an application may be used in any of a wide variety of circuits. The free end of the variable coil may be connected to a suitable terminal in a radio circuit and the metal case of the device may be grounded (or connected to another terminal) to provide a variable element in the circuit. The inductance in the circuit can then be varied by rotating the coil by the control knob. This varies the position of the contactor assembly with respect to the coil thus varying the inductance of the coil in the circuit.

The unused turns of coil M at the left of contactor 58 as seen in Figures 1 and 2 are shortcircuited, i. e., grounded to the case as follows: From the left-hand end of coil I0, through lug 53, ring 35, bushing 33,- shaft l2, contact 13 to the metal case. r I

A modified method of forming coil It) comprises first winding form It with the hard metal wire, such as hardened copper, and then plating the wire with silver or the like by immersing the assembly in a plating bath.

While the present invention, as to its objects and advantages, has been described herein as carried out in specific embodiments thereof, it is not desired to be limited thereby but it is intended to cover the invention broadly within the spirit and scope of the appended claims,

What is claimed is: s

1. A slide-wire variable inductance device wherein a bare helical coil is mounted for rotation about its longitudinal axis and a contact bar is mounted parallel to a side of the coil and spaced therefrom, characterized 'by the fact that there is combined with said coil and bar a moving contactor assembly, said contactor assembly comprising a frame, non-resilient means spacing said frame from said coil, a contact member on said frame and in contact with said coil and another contactmember on said frame and engaging said bar, the contact member engaging said bar comprising spring means urging said frame away from said contact bar and toward said coil.

2. A slide wire variable inductance device comprising a rotatable inductance coil member, a contactor carriage, a guide bar along-side said coil member guiding said carriage therealong, said carriage comprising a frame, a guide member supported thereon and guiding said carriage along the conductor of said coil member, and a contact on said frame guided thereby along said conductor in contact therewith, said frame following the inherent eccentricities in said coil member during rotation of said coil member, whereby the distance of said carriage from said bar is varied by said inherent eccentricities in said coil member during rotation thereof. I 3. A slide-wire variable inductance device comprising a rotatable inductive coil member, a contactor carriagea guide bar alongside said coil member guiding said carriage therealong, said carriage comprising a frame, a guide member supported thereon and guiding said carriage along the conductor of said coil member, and a contact on said frame guided thereby along said conductor in contact therewith, said frame following the inherent eccentricities in said coil member during rotation of said coil member, whereby the distance of said carriage from said bar is varied by said inherent eccentricities in said coil member during rotation thereof, the sole function of said carriage supported contact being to make contact with said wire.

4. A variable inductance device comprising a bare helical coil, means mounting said coil for rotation about its longitudinal axis, a contact bar parallel to a side of said coil and spaced therefrom, and a moving'contactor carriage interposed H directly between said coil and said contact bar, said contactor carriage comprising a frame, a guide member thereon engaging said coil so as to guide said carriage along the conductor of said coil during rotation thereof, a contact thereon engaging said coil and'held to slide thereon by said guide and frame, and contact spring means mounted on said carriage frame and compressed between said carriage and said contact bar and comprising the sole engagement between said carriage and said bar, said contact spring means making electrical contact between said bar and carriage and guiding said carriage'axially along sid coil.

5. A variable inductance device comprising an inductance coil of bare wire, a contactor assembly movable along the side of said coil, said assembly comprising an insulating trolley wheel and a sliding resilient metal contact, means for rotating said coil and means to guide said assembly along the side of said coil, said wire serving to guide said wheel and said wheel constraining said contact to slide along said wire, said contact serving solely to make continuous sliding contact'with said conductor.

6. A slide-wire variable inductance device wherein the useful inductance of a bare inductive coil is varied by moving a contact along the conductor of the coil, characterized by the fact that there is combined with said coil and contact a moving contactor carriage having a pair of insulating trolleys thereon guided by said conductor, said carriage holding said contact on said conductor and guiding said contact therealong.

'7. A slide-wire variable inductance device wherein the useful inductance of a bare inductive coil is varied by moving a contact along the conductor of the coil, characterized by the fact that there is combined with said coil and con-.

tact a moving contactor carriage having a pair of spaced insulating trolleys thereon guided by said conductor, said carriage holding said contact on said conductor at a point between said trolleys and guiding said contact therealong.

8. A variable inductance device comprising a cylindrical form and a bare conductor wound thereon to provide a helical coil, means supporting said form for rotation about its axis, a guide bar mounted parallel to the side of said form and a contactor carriage assembly mounted between said form and said bar, said contactor carriage assembly comprising an elongate frame extending parallel to the conductor of said helix and transverse to said bar, a pair of spring fingers secured to said elongate frame at an intermediate point and extending to spaced points of engagement with said bar whereby to make contact therewith and whereby to urge said carriage assembly away from said bar and toward said form, a pair of insulating trolley wheels pinioned at the ends of said elongate frame and grooved to said helical conductor whereby to travel along said conductor as a track when said coil and its form are rotated, and a sliding contact member secured to said elongate frame and having a bifurcated contacting portion thereof sliding along the conductor of said helical coil guided by said trolley wheels, said sliding contact member engaging said conductor along a path which is not touched by said trolley wheels in their travel along said conductor, said trolley wheels serving to determine the spacing between said carriage and said coil independent of eccentricity between said form and said guide bar, whereby substantially uniform contact pressure with said helical conductor is maintained.

9. A variable inductance device comprising a coil form of insulating material and a bare conductor wound thereon to form a coil unit, means to vary the inductance thereof comprising a contactor assembly to travel along said conductor as a track, said contactor assembly comprising a frame, two insulating trolley wheels pinioned therein and rollable on and guided by said'coil unit, one of said wheels preceding the other along the length of said conductor, and a resilient contact secured to said frame and slidable along said conductor, said trolley wheels thereby maintaining constant spacing between said frame and coil unit and serving to guide said contact along said conductor, a guide track positioned parallel to the side of said coil unit and in spaced relation thereto, said guide track and said coil unit being rotatable relative to each other about the axis of the coil unit, said contactor assembly being positioned between said guide track and the side of said coil unit, and resilient means engaging said contactor assembly and guide track and urging said contactor assembly against the side of said coil unit whereby said resilient means serves to absorb any variations in the spacing of said guide track and side of said coil unit due to eccentricities in the coil unit during relative rotation of the guide track and coil unit.

' 10. A variable inductance device comprising an inductance coil rotatable about its longitudinal axis, means for rotating said coil, 8. guide track positioned parallel to a side thereof, a contactor assembly comprising a frame, two insulating trolley wheels pinioned therein, and a bifurcated end contact spring attached to said frame, and a resilient spring attached to said frame and engaging said guide track, said assembly being interposed between said guide track and coil, the said wheels running on a turn of said coil as a track, said contact spring slidably pressing on said turn at a point adjacent said wheels and guided therealong by said wheels, said resilient spring being in compression between said'frame and the guide track whereby to hold the contactor assembly against the side of said coil, the pressure exerted on said turn by said bifurcated end contact spring being sufllcient to prevent both nibs jumping from said turn at the same time during travel of said contact along the coil and less than the pressure which would cause seizing.

11. A variable inductance device comprising an inductance coil rotatable about its longitudinal axis, means for rotating said coil, a guide track positioned parallel to a side thereof, a contactor assembly comprising a frame, two insulating trolley wheels pinioned therein, and a bifurcated end contact spring attached to said frame, and a resilient spring attached to said frame and engaging said guide track, said assembly being interposed between said guide track and coil, the said wheels running on a turn of said coil as a track, said contact spring slidably pressing on said turn at a point adjacent said wheels and guided therealong by said wheels, said resilient spring being in compression between said frame and the guide track whereby to hold the con- I tactor assembly against the side of said coil, the

spacing between the turns of the coil differing for different sections of the coil.

12. .5 variable inductance device comprising a coil form of insulating material, an inductance coil of bare wire wound thereon, a contactor engaging the conductor of said coil, a guide for said contactor comprising an insulating wheel rolling directly on said insulating form, means holding said contactor and wheel, said cell providing a guide for said trolley wheel, and means for moving said contactor and trolley wheel in unison along said conductor.

13. A variable inductance device comprising a coil form of insulating material, an inductive coil wound thereon, a contactor carriage, means to move said carriage along the conductor of said coil, said carriage comprising a frame, a wheel thereon rolling on said form and a contact resiliently supported thereon arranged to travel along said coil in contact therewith, whereby said wheel will follow any inundations and eccentricities in said form and said contact will follow the inundations and eccentricities in said conductor during travel of said carriage along said coil conductor.

14. A variable inductance device comprising a cylindrical form and a bare conductor. wound thereon as a helical coil, means supporting said form for rotation about its axis, a pair of guide bars mounted parallel to the'sides of said form and a contactor assembly mounted between said form and said bars, said contactor assembly comprising an elongate frame having a pair of sliding contacts at its ends bearing against contacting surfaces of said bars, a pair of trolley wheels riding pivoted to said frame and riding on said form guided by said conductor and a sliding contact member sliding along the length of said coil conductor.

15. A slide-wire variable inductance device wherein a coil unit made up of a form of circular cross section and an inductive coil of bare wire wound thereon is mounted for rotation about a longitudinal axis and a pair of spaced guide tracks are mounted parallel to the side of said coil unit in spaced relation thereto, characterized by the fact that there is combined with said coil ,unit and guide tracks a contactor assembly comprising an elongate frame member having a pair of trolleys longitudinally spaced thereon and engaging said coil unit and guided along said wire when said coil unit is rotated and a pair of spaced guide members on said frame engaging said guide tracks and guided therealong when said coil unit is rotated, the points of engagement of said guide members with said tracks and of said trolleys with said coil unit being aligned.

16. A slide-wire variable inductance device wherein 8000i] unit made up of a form of circular cross section and an inductive coil of bare wire wound .thereon is mounted for rotation about a longitudinal axis and a pair of spaced 1o guide tracks are mounted parallel to the side of said coil unit in spaced relation thereto, characterized by the fact that there is combined with said coil unit and guide tracks a contactor carr'iage assembly comprising an elongate frame 15 member having a pair of guide members at its ends bearing against said guide tracks, at least one of said guide members comprising a resilient member under compression between said frame and its respective guide track whereby to urge track and toward said coil unit, and at least 'one insulating trolley wheel pinioned at an intermediatepoint in said elongate frame and grooved to said wire whereby to travel along said wire as 25 a track when said coil unit is rotated, and a sliding contact member secured to said elongate frame and having a contacting portion thereof sliding along the wire of said coil when said coil said carriage assembly away from said guideunit is rotated, said trolley wheel guiding said sliding contact along said wire and determining the spacing between said carriage frame and said coil unit whereby the pressure of said contact on said wire is substantially independent of inherent 5 eccentricity between said coil unit and said guide tracks.

17. A slide-wire variable inductance device for tuning a radio frequency circuit comprising a bare conductor formed into a coil, a contactor assembly, means to move said contactor assembly along the conductor of said coil to vary the amount of said coil in the circuit being tuned, the sole contacting means between said contactor assembly and said conductor being a spring flnger, said contactor assembly comprising means maintaining uniform spacing with said coil and thereby maintaining uniform pressure between said spring finger and said conductor, the part of said spring finger slidably engaging said conductor comprising a pair of contact nibs sliding in parallel along the length of the conductor whereby the length of said conductor in the circuit at any setting of the device is the same with either nib in contact with the conductor and the other nib lifted off.

- PAUL WARE. 

